Microlithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely structured components, e.g. masks for microlithography. These methods involve the use of a mask (reticle) that bears the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object plane of the projection lens and is illuminated with an illumination radiation provided by the illumination system. The radiation changed by the pattern passes as projection radiation through the projection lens, which images the pattern onto the substrate which is to be exposed and is coated with a radiation-sensitive layer and whose surface lies in the image plane of the projection lens, the image plane being optically conjugate with respect to the object plane.
In order to be able to produce ever finer structures, in recent years optical systems have been developed which operate with moderate numerical apertures and obtain high resolution capabilities substantially via the short wavelength of the electromagnetic radiation used from the extreme ultraviolet range (EUV), in particular with operating wavelengths in the range of between 5 nm and 30 nm. In the case of EUV lithography with operating wavelengths around 13.5 nm, for example given image-side numerical apertures of NA=0.3, theoretically a resolution of the order of magnitude of 0.03 μm can be achieved with typical depths of focus of the order of magnitude of approximately 0.15 μm.
Radiation from the extreme ultraviolet range cannot be focused or guided with the aid of refractive optical elements, since the short wavelengths are absorbed by the known optical materials that are transparent at higher wavelengths. Therefore, mirror systems are used for EUV lithography.
In the field of EUV microlithography, too, endeavours are made to further increase the resolution capability of the systems used by developing projection systems having an ever higher image-side numerical aperture NA, in order to be able to produce ever finer structures. For a given imaging scale β, the object-side numerical aperture NAO thus increases as well.
For higher-aperture EUV systems, narrowband masks pose a challenge because their reflectivity capability decreases greatly at larger angles of incidence of the radiation. Therefore, it has already been proposed to use greater reductions instead of the customary reducing imaging scale of 1:4 (|β|=0.25) for lithographic-optical systems. By way of example, an imaging scale of 1:8 (|β|=0.125) instead of 1:4 (|β|=0.25) halves the object-side numerical aperture NAO and thus also the angles of incidence of the illumination radiation at the mask by half. However, this imaging scale (for the same mask size) reduces the size of the exposed field and thus the throughput.
It has also already been recognized that when the object-side numerical aperture is increased, the object-side principal ray angle is increased, which can lead to shading effects by the absorber structure of the mask and to problems with the layer transmission. In particular, severe apodization effects can occur owing to the reticle coating, as discussed, for example, in WO 2011/120821 A1.
WO 2012/034995 A2 proposes for this reason, inter alia, designing an EUV projection lens as anamorphic projection lens. An anamorphic projection lens is characterized in that a first imaging scale in a first direction deviates from a second imaging scale in a second direction perpendicular to the first direction. The deviation lies significantly outside deviations possibly caused by manufacturing tolerances.
An anamorphic projection lens enables e.g. a complete illumination of an image plane with a large object-side numerical aperture in the first direction, without the extent of the reticle to be imaged in the first direction having to be increased and without the throughput of the projection exposure apparatus being reduced. Furthermore, in comparison with systems having a uniform imaging scale in both directions, a reduction of the losses of imaging quality that are caused by the oblique incidence of the illumination light can also be obtained.
If a 1:8 imaging scale (|β|=0.125) is set e.g. in the scan direction, where the field extent is small, while the customary 1:4 imaging scale (|β|=0.25) acts perpendicularly to the scan direction (cross-scan direction), then this does not introduce particularly large angles at the mask, but ensures that the field size compared with conventional non-anamorphic projection lenses with |β|=0.25 in both directions is only halved and not quartered. Moreover, the option arises of achieving full field again with larger reticles.
A projection exposure apparatus generally includes a manipulation system having a multiplicity of manipulators that make it possible to change the imaging properties of the system in a defined manner on the basis of control signals of a control unit. In this case, the term “manipulator” denotes, inter alia, optomechanical devices designed for actively influencing individual optical elements or groups of optical elements on the basis of corresponding control signals, in order to change the optical effect of the elements or groups in the projection beam path. Often, manipulators are also provided in order for example to displace, to tilt and/or to deform the mask and/or the substrate. Generally, manipulators are set in such a way that metrologically detected imaging aberrations can be reduced in a targeted manner.
In some EUV systems, a displacement of the reticle with components perpendicular to the object plane and/or a tilting constitute(s) an effective manipulation possibility in order to correct imaging aberrations. With oblique incidence of radiation in the reflective reticle and/or in non-telecentric systems, it is also possible to correct a lateral offset of structures via such reticle displacements. In this case, the active principle is based on the fact that, with non-telecentric illumination, a z-defocusing of the reticle alongside a corresponding z-defocusing of the image always additionally results in a lateral shift of the image. If the telecentricity of the illumination within the object field varies e.g. quadratically, then there is also a quadratic variation of the lateral image shift in the case of z-decentration, which can be used e.g. to correct quadratic distortion profiles present on the reticle or the substrate.
DE 10 2004 014 766 A1 (cf. U.S. Pat. No. 7,372,539 B2) proposes, for the purpose of correcting anamorphism in a projection lens of an EUV projection exposure apparatus, tilting the reticle by a small angle about an axis that is perpendicular to the axis of the projection lens and perpendicular to the scan direction and in each case is situated through the center of the light field generated on the reticle or the wafer.
EP 1 039 510 A1 proposes adjusting and tilting the reticle in the direction of the optical axis in order to correct aberrations in the imaging scale and the position of the generated image.